The present invention relates to an engine integrated auxiliary power unit for use on an aircraft.
FIG. 1 shows a schematic of a modern, high-bypass, three-spool turbofan engine 10. Air enters the engine 10 through an inlet diffuser (not shown) and reaches the fan 12, which acts as a low pressure compressor and which is driven by a three-stage low turbine 14. Downstream of the fan 12, a part of the air is compressed fully and passed through the intermediate pressure compressor 16, which is driven by the intermediate turbine 18, and then through the high pressure compressor 20, which is driven by the high pressure turbine 22, into the combustion chamber 24. The remainder of the airflow bypasses the combustion chamber 24 to provide cold thrust. The bypass flow rejoins the remainder of the hot flow downstream of the turbine 14.
Typical two-spool arrangements combine the fan with the first intermediate compressor stages into the low pressure compression system, whereas the remaining stages are combined with the high pressure compressor. The turbine work split between the high pressure and low pressure systems is adjusted accordingly.
In either case, jet engine accessories like electrical power generators and pumps are driven via bevel gears, tower shafts, and external gearbox from the high pressure shaft. For engine start, the same arrangement is used to drive the high pressure spool from a starter, which expands pressurized air in a starter turbine. In order to be able to start the engine with such an arrangement, pressurized air needs to be provided and ducted to the engine. This is one of the major tasks for the auxiliary power unit, a small gas turbine that is usually located in the tail-cone of an aircraft. Additional functions of the auxiliary power unit include generation of electrical and shaft power while the main engines are not running, e.g. at the gate, as well as providing air for the aircraft's environmental control system.
From an aircraft system point of view, potential weight and cost savings are seen in a co-location of auxiliary power unit and main engines. Such an engine mounted auxiliary power unit would eliminate heavy bleed air ducting, reduce parasitic losses, and simplify transmission systems. It also may offer more attractive thermodynamic cycles. To date, however, no one has developed a truly engine integrated auxiliary power unit.